Reference may be made to U.S. Pat. No. 4,381,719 which issued May 3, 1983 and to the corresponding German open application DE-OS No. 30 18 797.
These documents describe a method of and a positioning device for correcting at least some of the stitches of a given stitch row, utilizing stitch counting, so that the stitch row will end at a predetermined point of a fabric workpiece.
That system operates with a sewing machine which, apart from the correction means, may be of conventional design, i.e. can have a worktable over which the workpiece, e.g. a garment, can be moved and which is overhung by the head of the sewing machine which is carried by the sewing machine arm. The latter, in turn, extends horizontally toward a stitching location from the post of the sewing machine.
The head is usually provided with an upright needle bar which can be displaced upwardly and downwardly, e.g. by an arm mechanism so that a needle carried by this bar will pierce the workpiece and produce a row of stitches therein as the workpiece is moved past the stitching location by fabric-feed dogs or some other transport device usually provided beneath the workpiece adjacent the stitching location.
Such fabric feed dogs are given a compound movement, e.g. by a shaft under the table, and connected to the arm shaft through a mechanism in the post which may be driven by an electric motor. Usually the dog rises through a slot in a stitch plate which is set into the worktable and, in engagement with the underside of the fabric, moves in a fabric-feed direction before it retracts beneath the surface of the stitch plate to rise again for another increment of displacement of the workpiece.
While other mechanisms may be provided on the machine, including thread tensioning devices and means for feeding an upper thread to the needle and for drawing the thread taut to ensure tight stitching, means for correlating the motion of the dog with that of the needle to produce stitches of predetermined or adjustable length and means below the stitch plate to assist in stitch formation, the only other part of a standard machine which is necessary for an understanding of this invention is the presser foot which is also mounted in the head of the arm, generally straddles the stitching location, and yieldably holds the workpiece against the fabric feed dog so that a practically positive engagement of the fabric is ensured for the advance thereof.
It is important, for reasons discussed in the aforementioned U.S. patent and the German open application, to be able to terminate a given row of stitches at a predetermined point in the workpiece, e.g. at a predetermined distance from a fold or edge. A typical case is the sewing of a label on a garment.
To this end it is known to provide a photoelectric detection of the fabric edge or fold and to provide, on the arm shaft of the sewing machine, on which the needle bar cam may be mounted, appropriate markings which can be photoelectrically sensed to generate a train of pulses representing the angular displacement of the arm shaft and hence the position of the needle and the number of stitches produced by the successive up-and-down displacements of the needle bar.
A counting device which can include a counter responds to the photoelectric signals to compensate in the row of stitches so as to bring about a termination of this row of stitches at a predetermined time or after the row has been stitched through a predetermined distance, thereby ensuring that the last penetration of the needle in a stitch-forming mode will be at the predetermined location with respect to the photoelectrically detected reference, namely, the edge or the fold.
In the system described in this patent and the open application, the arm shaft has two marks which are photoelectrically detected and which each extend over 180.degree., one of these marks being reflective and the other being nonreflective. The output of the photoelectric detector is applied to a stitch counter and depending upon the position of the shaft, a determination is made as to whether n additional stitches should be generated to the end of the row or whether this row should be limited to n-1 stitches, where n is of course an integer.
In this system each stitch row will end at the predetermined location with a tolerance or variation of .+-. one-half stitch length. Stated otherwise, if a set point location is established, the actual end of the stitch row will be at a maximum .+-. half of a stitch length from the set point.
While this may not seem like much of a deviation, it represents a high degree of imprecision which is undesirable in accurate sewing.
It has also been proposed to provide a control system for a sewing machine which permits more exact row length and more precise terminal needle penetrations to be ensured. Such sewing machines are described, for example, in the open Europatent application EP-OS No. 0 044 648. This sewing machine utilizes a variable speed electric motor which is controlled by a counting device for mounting the number of sewn stitches and with sensors for the workpiece edge. The microprocessor-controller establishes the last needle penetration by breaking down the count into increments and reducing the speed of the sewing machine so that a programmed number of stitches at the reduced speed are sewn to a predetermined end of the row.
An adaptive algorithm can be used to increase the precision by varying the row length because with this system it is possible to reduce the stitch length so that, for example, a terminal stitch of half the stitch length can be sewn.
In many stitching applications, especially for decorative rows of stitching, a terminal stitch which is only half as long as the other stitches is undesirable and the problem is more pronounced as the selected stitch length is greater. This control system is also comparatively expensive.